[Characteristics of soil organic carbon components across a chronosequence of <i>Cryptomeria japonica</i> plantations in Rainy Area of Western China].

Soil organic carbon (SOC) components are crucial indicators of soil fertility and carbon sequestration capacity, playing a pivotal role in maintaining global carbon balance. However, the dynamics of SOC components across plantations with different stand ages remain poorly understood. In this study, we collected soil samples (0-15 cm and 15-30 cm) from 7, 13, 24, 33 and 53 years stands of <i>Cryptomeria japonica</i> plantations in the Rainy Area of Western China to quantify the contents of different SOC components, including particulate organic carbon (POC), easily oxidized organic carbon (EOC), light fraction organic carbon (LFOC), heavy fraction organic carbon (HFOC), labile organic carbon (LOC), and inert organic carbon (IOC). The results showed that: 1) POC content increased continuously with stand age, while contents of LOC, EOC, LFOC, HFOC, and IOC followed unimodal patterns, peaking at 24- and 33-year-old stands. 2) All SOC components were significantly higher in surface layer (0-15 cm) than subsurface layer (15-30 cm). The differences in POC and LFOC between two soil layers increased with stand ages, while EOC and LOC exhibited maximal inter-layer differences at 24 and 33 years, respectively. 3) Results of variance partitioning analysis showed that microbial variables (fungal and bacterial biomass, and enzyme activities) exerted stronger explanatory power on POC variation than soil physicochemical pro-perties. Other SOC components were co-regulated by both biological and physiochemical factors. Partial least squares path modeling further demonstrated that stand age directly influenced POC and indirectly modulated all SOC components by mediating soil physicochemical and microbial characteristics. In all, our findings suggested that there were distinct age-dependent patterns in SOC components in <i>C. japonica</i> plantation and their vertical stratification were driven by stand age-associated changes in soil microbial and physiochemical properties. This study would be helpful for understanding soil carbon sequestration along plantation development, and could offer scientific basis for enhancing carbon sink functions in <i>C. japonica</i> plantation.